1. Field of the Invention
This invention relates to a method of aligning two objects, for example, two glass plates for liquid crystal.
2. Description of the Prior Art
Generally, transparent electrodes are used in various display plates such as liquid crystal display plates, and the usage of these display plates ranges over a wide field such as the dials of optical instruments, electronic instruments, timepieces, etc., large displays for liquid crystal televisions and thermometers. Two substrates for enclosing a material such as liquid crystal are used in such a display plate and each of these substrates is formed with a transparent electrode, and whether the patterns of these transparent electrodes are accurately in accord with each other is important in respect of the performance of the product.
Heretofore, such kind of alignment has been accomplished by printing an alignment mark M1 on one substrate and printing a similar mark M2 on the other substrate as shown in FIG. 1 of the accompanying drawings, and judging that alignment of the two substrates has been completed if the mark M2 enters into the mark M1 as shown in FIG. 1, but this judgment has been done by visual observation and accordingly, the work efficiency has been low and there has been a hindrance to the accuracy of alignment.
On the other hand, in a semiconductor printing apparatus, as a step before printing the semiconductor integrated circuit pattern of a mask on a wafer, there is the step of accurately aligning the mask and the wafer.
In the conventional alignment step, linear alignment marks are formed on the mask and the wafer in advance, and these marks are scanned by a laser beam and the scattered light from the marks is detected to thereby measure the relative positional deviation of the wafer and the mask. The wafer is then moved by the amount of this measured positional deviation, whereby accurate alignment of the mask and the wafer is effected. However, linear marks are generally hard to roughly adjust as compared with area type marks, and their detection output is usually weak and therefore, linear marks are liable to be affected by noise such as dust which may cause a reduction in output.